A favoured technique for detecting radiation is through the use of a photographic element containing a silver halide emulsion. Such an element is usually exposed with light (which includes infra-red and ultraviolet as well as visible light) in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum.
It is well known that to use sensitizing dyes to sensitize the silver halide to other than the blue region (for example, to other areas of the visible spectrum or to infrared light). Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds). Their usual function is to adsorb to the silver halide and to absorb light (usually other than blue light) and transfer that energy via an electron to the silver halide grain thus, rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity. However, sensitizing dyes can also be used to augment the sensitivity of silver halide in the blue region of the spectrum. The resulting sensitized emulsion will generally have a sensitivity versus wavelength curve with a peak sensitivity the same as, or close to, the wavelength of peak absorption of the sensitizing dye used. The sensitivity curve will then fall off fairly rapidly on either side of the peak sensitivity wavelength.
One particular application of photographic elements in the form of paper or film, is for recording the output from devices such as laser printers which are designed to reproduce black and white or color digitized photographic images. Those printers operate by scanning a photographic element with a controlled laser beam modulated in accordance with the digital image. Following exposure, the photographic element is developed in the same manner as other photographic materials. Typically, the laser beam is in the infra-red region, for example 780nm, and is generated by laser diodes. Any color couplers incorporated in such a silver halide emulsion therefore produce a false-color image.
A difficulty with printers of the foregoing type is that the laser diodes vary in their output wavelength from diode to diode. For example, manufacturers may specify the wavelength of a monochrome printer being 780 nm.+-.20 nm. Thus, there may be variation in wavelengths within a given printer using multiple diodes, as well as between printers. Since the sensitized emulsion has a wavelength dependent sensitivity curve as described above, the undesirable result is that the intensity of a given point to be recorded on the photographic element may vary from diode to diode and printer to printer. In view of this situation, IR sensitive films and papers used in such printers desirably have a constant photographic response over a range of wavelengths so as to provide invariable results in printers using an array of laser diodes, or from printer to printer.
One method for providing a broad sensitivity in the infrared region of the spectrum is described in U.S. Pat. No. 5,013,642 to Muenter et al. The Muenter et al. patent describes the use of a combination of sensitizing dyes with maximum sensitivities differing by between about 5 to 100 nm. It would be desirable though, to provide a radiation detecting element, particularly a silver halide photographic element useful in printers of the type described, which has a decreased change in sensitivity over a given wavelength range.